"dash" is a POSIX compliant shell that is much smaller than "bash".
Dash supports many features that a real sh shell would support, however
it is much smaller in size. This becomes an advantage in situations
where there is a lack of memery (initial ram-disks, etc). dash does
lack a few features, like command line history.
dash is the continuation of the original NetBSD ash fork. dash is much
more up-to-date, and properly maintained.
You should install dash if you need a near featureful lightweight shell
that is similar to GNU's bash.